Under this plan, signed by both Syria's ally Iran and Assad's pro-rebel foe Saudi Arabia, a nationwide ceasefire will precede Geneva-based talks on "political transition".

But there has been little progress towards a resumption of talks that had been expected to take place this month.

And the prospects for a political transition beginning by August, as laid out in the plan, now appear slim.

Kerry was due to arrive later on Thursday in Moscow, a close ally of Assad's government that launched air strikes in support of regime forces last September.

Kerry said before leaving Washington that he would meet Putin "to see if we can somehow advance this [the peace process] in the important ways that people want us to".

The Washington Post reported on Thursday that Washington was to offer to cooperate with Russia in joint military action against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) and the Syrian branch of al-Qaeda, al-Nusra Front.

In Paris before heading to Moscow, Kerry did not deny the report, but refused to discuss the proposal in detail until he had been to the Kremlin.

Sergey Karagonov, a former adviser to the Russian president, said: "The problem is that he [Kerry] represents a lame duck. The general mood in the US is very negative towards Russia, towards cooperation."

He added that while Lavrov and Kerry did not want to "exacerbate things together ... it does not look like the problem will be solved easily".

White House spokesman Josh Earnest said on Thursday that while there was "some speculation that an agreement may be reached", it was "not clear that that will happen".

"At present, the United States is not conducting or coordinating military operations with Russia," he said.

Kerry's visit to Moscow came amid fresh concerns over food aid in Syria. The UN on Wednesday said that along with other aid agencies, it has enough food in rebel-held eastern Aleppo to feed 145,000 people for one month, as pro-government forces continued to make progress on encircling and besieging the area, which has a population of as many as 300,000 people.

Kerry 'extremely frustrated'

Moscow and Washington brokered a landmark partial ceasefire in Syria in February, but it has since all but collapsed amid continued heavy fighting.

Kerry's spokesman John Kirby told reporters his boss was "extremely frustrated" with the failure of peace efforts and "his patience was growing thin".

In Washington, many observers have criticised Kerry's outreach to Russia on Syria, arguing he has been strung along by Putin as the latter seeks to protect his client Assad.

But Kirby insisted the administration is not being naive, and that Thursday's visit to Moscow, Kerry's third this year, would "probe the sincerity" of Putin's promises.

Syria's conflict began in 2011 with the repression of anti-government demonstrations and has evolved into a complex multi-front war that has killed more than 280,000 people and forced millions from their homes.

Efforts to bring an end to the war have taken on greater urgency since the emergence of ISIL, which seized control of large parts of Syria and neighbouring Iraq in mid-2014.

The group has committed widespread atrocities in areas under its control and organised or inspired a wave of attacks across the Middle East and in Western cities.

A US-led coalition is carrying out air strikes against ISIL fighters in Syria and Iraq and recent months have seen ISIL lose a significant amount of territory.

Colvin 'responsible' for her own death

According to the Post, which cited sections of what it said was a draft agreement, US and Russian commanders would set up a joint command and control centre to direct intensified air strikes against the groups.

Such a deal is likely to face criticism that it amounts to a tacit acceptance of Putin's efforts to shore up Assad's regime.

In his interview with NBC, Assad also said a US reporter killed in alleged Syrian government bombardment in 2012 was responsible for her own death.

Marie Colvin, a 56-year-old war correspondent working for British newspaper The Sunday Times, died in the rebel-held Baba Amr district of Syria's third city Homs.

"It's a war and she came illegally to Syria. She worked with the terrorists, and because she came illegally, she's been responsible of everything that befall on her," Assad said, speaking in English.

Asked if she was responsible for her own death, Assad replied "of course", though he denied that his forces had targeted her.